1. Field of the Invention
The invention relates generally to the field of seismic surveying systems and techniques. More specifically, the invention relates to arrangements for mounting particle motion sensors used with marine seismic streamers.
2. Background Art
In seismic exploration, seismic data are acquired by imparting acoustic energy into the earth near its surface, and detecting acoustic energy that is reflected from boundaries between different layers of subsurface earth formations. Acoustic energy is reflected when there is a difference in acoustic impedance between layers disposed on opposite sides of a boundary. Signals representing the detected acoustic energy are interpreted to infer structures of and composition of the subsurface earth structures.
In marine seismic exploration, (seismic exploration conducted in a body of water) a seismic energy source, such as an air gun, or air gun array, is typically used to impart the acoustic energy into the earth. The air gun or air gun array is actuated at a selected depth in the water, typically while the air gun or air gun array is towed by a seismic survey vessel. The same or a different seismic survey vessel also tows one or more seismic sensor cables, called “streamers”, in the water. Generally the streamer extends behind the vessel along the direction in which the streamer is towed. Typically, a streamer includes a plurality of pressure sensors, usually hydrophones, disposed on the cable at spaced apart, known positions along the cable. Hydrophones are sensors that generate an optical or electrical signal corresponding to the pressure of the water or the time gradient (dp/dt) of the pressure in the water. The vessel that tows the one or more streamers typically includes recording equipment to make a record, indexed with respect to time, of the signals generated by the hydrophones in response to the detected acoustic energy. The record of signals is processed, as previously explained, to infer structures of and compositions of the earth formations below the locations at which the seismic survey is performed.
Marine seismic data often include ghosting and water layer multiple reflections, because water has a substantially different acoustic impedance than the air above the water surface, and because water typically has a substantially different acoustic impedance than the earth formations below the bottom of the water (or sea floor). Ghosting and water layer multiples can be understood as follows. When the air gun or air gun array is actuated, acoustic energy radiates generally downwardly where it passes through the sea floor and into the subsurface earth formations. Some of the acoustic energy is reflected at subsurface acoustic impedance boundaries between layers of the earth formations, as previously explained. Reflected acoustic energy travels generally upwardly, and is ultimately detected by the seismic sensors on one or more streamers. After the reflected energy reaches the streamers, however, it continues to travel upwardly until it reaches the water surface. The water surface has nearly complete reflectivity (a reflection coefficient about equal to −1) with respect to the upwardly traveling acoustic energy. Therefore, nearly all the upwardly traveling acoustic energy will reflect from the water surface, and travel downwardly once again, where is may be detected by the sensors in the streamer. The water-surface reflected acoustic energy will also be shifted in phase by about 180 degrees from the upwardly traveling incident acoustic energy. The surface-reflected, downwardly traveling acoustic energy is commonly known as a “ghost” signal. The ghost signal causes a distinct “notch”, or attenuation of the energy within a particular frequency range.
The downwardly traveling acoustic energy reflected from the water surface, as well as acoustic energy emanating directly from the seismic energy source, may reflect from the water bottom and travel upwardly, where it can be detected by the sensors in the streamer. This same upwardly traveling acoustic energy will also reflect from the water surface, once again traveling downwardly. Acoustic energy may thus reflect from both the water surface and water bottom a number of times before it is attenuated, resulting in so-called water layer reverberations. Such reverberations can have substantial amplitude within the total detected acoustic energy, masking the acoustic energy that is reflected from subsurface layer boundaries, and thus making it more difficult to infer subsurface structures and compositions from seismic data.
So-called “dual sensor” cables are known in the art for detecting acoustic (seismic) signals for certain types of marine seismic surveys. One such cable is known as an “ocean bottom cable” (OBC) and includes a plurality of hydrophones located at spaced apart positions along the cable, and a plurality of geophones on the cable, each substantially collocated with one of the hydrophones. The geophones are responsive to the velocity of motion of the medium to which the geophones are coupled. Typically, for OBCs the medium to which the geophones are coupled is the water bottom or sea floor. Using signals acquired using dual sensor cables enables particularly useful forms of seismic data processing. Such forms of seismic data processing generally make use of the fact that the ghost signal is substantially opposite in phase to the acoustic energy traveling upwardly. The opposite phase of the ghost reflection manifests itself by having opposite sign or polarity in the ghost signal as compared with upwardly traveling acoustic energy in the signals measured by the hydrophones, while the geophone signals are substantially the same polarity because of the phase reversal and reversal of the direction of propagation of the seismic energy. While OBCs provide seismic data that is readily used to infer subsurface structure and composition of the Earth, as their name implies, OBCs are deployed on the water bottom. Seismic surveying over a relatively large subsurface area thus requires repeated deployment, retrieval and redeployment of OBCs.
One type of streamer, including both pressure responsive sensors and particle motion responsive sensors is disclosed in U.S. Pat. No. 7,239,577 issued to Tenghamn et al. and assigned to the assignee of the present invention, incorporated herein by reference. So called “dual sensor” streamers make possible the use of a technique for attenuating the effects of ghosting and water layer multiple reflections as disclosed, for example, in U.S. Pat. No. 7,123,543 issued to Vaage et al., and assigned to the assignee of the present invention.
Seismic streamers, because they are towed in the water, are subject to various types of motion in the water, other than the motion imparted by the tow vessel. Particle motion sensors in a streamer respond not only to seismic energy induced motion of the water, but to motion of the streamer cable itself, which motion may be induced by sources other than seismic energy propagating through the water. Motion of the streamer cable may include mechanically induced noise along the streamer cable, among other sources. Such cable motion unrelated to seismic energy may result in noise in the output of the particle motion sensors which may make interpretation of the seismic signals difficult. It is desirable, therefore, to provide a streamer cable having motion sensors arranged in a manner that reduces cable noise coupled into the motion sensors, while substantially maintaining sensitivity of the particle motion sensors to seismic energy.